In gas turbine applications, blades and vanes face vibratory excitation during operation. This excitation causes stresses in parts. These stresses must be kept below certain limits to avoid premature failure of the part or gas turbine, respectively.
One means to keep stresses below limits is to increase damping by introduction of damping devices. Commonly, interlocking shrouds are used to this end. These shrouds provide besides a damping effect also a frequency-tuning mean. However, in some cases this is not sufficient e.g. for very long airfoils or not possible for shroud-less blades/vanes.
Thus, additional damping devices are used like under platform dampers as disclosed for example in U.S. Pat. No. 5,369,882, pendulum absorbers as disclosed for example in U.S. Pat. No. 5,924,845 or impact dampers as disclosed for example in U.S. Pat. No. 6,827,551, the contents of all of which are incorporated herein by reference as if fully set forth.
U.S. Pat. No. 5,924,845 states, that the pendulum absorber becomes more robust with respect to vibration attenuation and lifetime if it is damped. It is noted that this damping comes from some aerodynamic damping within the hollow blade and due to friction in the attachment of the pendulum. However, these types of damping are limited to a narrow range of damping ratios ξP.
Impact dampers can also increase the damping and thus reduce the vibratory stresses in parts. Such an arrangement is shown in U.S. Pat. No. 6,827,551 disclosing blades with impact damper arrangements. The added damping Δξ by an impact damper depends, in addition to other parameters, on the viscous damping ratio ξ2 between primary mass (e.g. blade or vane in which the corresponding cavity is located) and the damper mass (damping element, ball).